Most present day drilling rigs use the same basic approach for sinking well casings down the drilled hole. Sections of the casing are fitted together at the ground surface and then driven at the top of the drill hole downwardly into the hole as the drilling progresses. Different methods are used for driving the casing but the basic driving technique at the top of the drill hole remains the same.
Placing a long string of well casing sections under high compressional forces has several drawbacks. Frictional resistance to downward movement increases as the hole depth increases, therefore correspondingly increasing the requirements for casing driving forces. Long drill strings have a tendency to buckle similar to a long column under compression. The surrounding earth prevents such buckling at the cost of increased friction against the sides of the drilled hole. A casing driven from the top of the hole will normally follow the drilled hole but not with a desirable degree of accuracy, especially in soft ground. Section welds can easily become damaged, due to constant lateral shifting (partial buckling), when under compressive forces from above.
The above problems are recognized to a limited degree by Davey, Sr., et al, U.S. Pat. No. 3,190,378 granted June 22, 1965. The Davey, Sr. case driving mechanism makes use of apparatus for drilling and for pulling casing downwardly into a drilled hole. A rotary drill bit is releasably connected to a casing shoe mounted to the bottom of the casing. As the rotary drill bit rotates, L-shaped brackets on the rotary drill bit engage dogs projecting inwardly from the casing shoe. The casing shoe is rotatably mounted to the bottom of the casing and rotates with the rotary drill bit. The rotary drill bit rotates the casing shoe and additionally pulls the casing downwardly as the drilling progresses. At the end of the drilling operation, the drill tool is rotated in an opposite rotational direction to disengage the rotary drill bit and the inverted L-shaped brackets from the dogs, thus enabling the retraction of the rotary drill bit and the drill string up through the casing.
A cutting shoe of the Davey, Sr. equipment is extremely expensive. The shoe must have specially hardened and formed drill teeth at lower ends and appropriate sealed bearings at upward ends where the shoe is connected to the casing bottom to enable the shoe to rotate and perform part of the drilling function. The cutting shoe serves as a secondary bit which is left in the hole. Should the bearing fail or freeze, the shoe will twist the casing as the rotating bit rotates. Such a failure would require the entire string to be removed from the drilled hole for repair.
A further disadvantage of the Davey, Sr. device is the difficulty of flushing the drilled earth material from the hole since the flushing fluid has a tendency to migrate into the surrounding earth or up the outside of the casing without removing the drilled material up the inside of the casing.
A pile driving device is shown in the Blumenthal U.S. Pat. No. 1,908,217 granted May 9, 1933 wherein a drive point is hammered into the ground by a downhole pile driver. The pile shell is pulled downwardly by the downhole pile driver. The Blumenthal device is used exclusively for driving pilings and does not suggest use in a drilling operation in which earth material is to be removed from the hole. Blumenthal, however, exemplifies the desirability for downhole "driving" of a piling shell to prevent compressive damage of the piling shell and to decrease the force required to move the piing shell down the hole.
The present invention overcomes many of the problems of the prior art device and greatly increases the efficiency of drilling and driving casing into a bore hole.